1. Field of the Invention
This invention relates to osmotic delivery devices for delivering beneficial agents, and more particularly, to osmotic delivery devices having an osmotic engine and a valve to prevent expulsion of the beneficial agents.
2. Description of the Related Art
Controlled delivery of beneficial agents, such as drugs, in the medical and veterinary fields has been accomplished by a variety of methods. One method for controlled prolonged delivery of beneficial agents involves the use of osmotic delivery systems. These systems can be implanted within a body of a human or animal to release beneficial agents in a controlled manner over a preselected time or administration period. In general, osmotic delivery systems operate by imbibing liquid from the outside environment and releasing corresponding amounts of the beneficial agent.
A known osmotic delivery system, commonly referred to as an xe2x80x9cosmotic pump,xe2x80x9d generally includes some type of a capsule or enclosure having a semipermeable portion which selectively passes water into an interior of the capsule containing a water-attracting osmotic agent. In one known osmotic delivery system the walls of the capsule are substantially impermeable to items within and outside the capsule. A membrane plug is inserted into one end of the capsule and acts as the semipermeable portion allowing water to pass into the interior of the capsule. The difference in osmolarity between the water-attracting osmotic agent and the environment surrounding the capsule causes water to pass through the membrane plug into the capsule which in turn causes the beneficial agent within the capsule to be delivered through a delivery orifice. The water-attracting osmotic agent may be the beneficial agent delivered to the patient; however, in most cases a separate osmotic agent is used specifically for its ability to draw water into the capsule.
When a separate osmotic agent is used, the osmotic agent may be separated from the beneficial agent within the capsule by a movable dividing member or piston. The structure of the capsule is such that the capsule does not expand when the osmotic agent takes in water and expands. As the osmotic agent expands, it causes the piston to move and the beneficial agent to be discharged through the delivery orifice at the same rate as the liquid, which is typically water, enters the osmotic agent by osmosis. Osmotic delivery systems may be designed to deliver a beneficial agent at a controlled constant rate, a varying rate, or in a pulsatile manner.
In the known osmotic delivery systems, an osmotic tablet is generally used as the osmotic agent and is placed inside the capsule adjacent the piston. A membrane plug is placed in an opening in the capsule through which the tablet and piston were inserted. Known membrane plugs are typically cylindrical members which seal the interior of the capsule from the exterior environment, permitting only certain liquid molecules from the environment of use to permeate through the membrane plug into the interior of the capsule. The rate that the liquid permeates through the membrane plug controls the rate at which the osmotic agent expands and drives the beneficial agent from the delivery system through the delivery orifice. The rate of delivery of the beneficial agent from the osmotic delivery system may be controlled by varying the size of the beneficial agent delivery orifice, the osmotic material, a size and shape of the membrane plug, or the permeability coefficient of the membrane plug.
It is desirable to seal the beneficial agent delivery orifice of the delivery system to prevent incursion of materials into the delivery system before sufficient osmotic pressure exists to insure a flow of the beneficial agent through the orifice. Protecting the beneficial agent from the external environment is particularly important when the beneficial agent is a protein formulation or other agent which breaks down when in contact with certain environmental compositions.
In order to prevent contamination or early release of the beneficial agent, some delivery systems are provided with a plug in the orifice which is discharged upon movement of the piston by the fluid pressure within the system. Typically, such osmotic delivery systems use mechanical plugs, bio-eroding, or dissolving plugs.
With mechanical plugs, such plugs are chemically stable materials discharged from the delivery system on movement of a piston contained within the system. Premature release of the beneficial agent may occur when the delivery system is jarred, thereby loosening the mechanical plug from the system. Further, mechanical plugs expelled from the delivery device may not be acceptable with the patient when left in the patient""s body at the implant site.
Bio-eroding or dissolving plugs also present drug delivery problems since such plugs allow the drug delivery orifice to open regardless of whether or not the osmotic agent can exert sufficient hydraulic pressure to insure flow of the beneficial agent.
Because of the above-identified problems associated with current osmotic delivery systems, it is desirable to prevent contamination of the beneficial agent and to prevent beneficial agent leakage by providing a delivery orifice valve which is not expelled into the patient""s body.
The present invention relates to osmotic delivery systems having an osmotic engine and a valve to prevent contamination and/or expulsion of the beneficial agents.
In accordance with one aspect of the present invention, a delivery system for controlled delivery of a beneficial agent includes an implantable capsule having a delivery orifice, a separating member dividing the capsule into a beneficial agent reservoir and a driving reservoir, an osmotic engine in the driving reservoir, and a valve member that can move from a closed position to an open position. In the closed position, the valve member prevents the expulsion of beneficial agent from the beneficial agent reservoir through the delivery orifice. The implantable capsule can include an attachable cap having a vent. In operation, the osmotic engine imbibes fluid thereby causing the engine to swell. This swelling causes the osmotic engine to exert a pressure on the separating member whereby such pressure moves the separating member, the beneficial agent reservoir, and the valve member a distance such that the valve member moves to an open position, allowing passage of beneficial agent through the delivery orifice at a desired delivery rate.
In accordance with another aspect of the present invention, a method of preventing contamination from entering the osmotic delivery device before activation includes the steps of providing a delivery device capsule enclosing a first chamber which contains a beneficial agent and a valve member. The first chamber has an opening communicating with the external environment. Before activation of the delivery device, the valve member occludes the opening. This occlusion prevents the beneficial agent from leaving the device, as well as prevents the incursion of contaminants into the device.
In accordance with an additional aspect of the present invention, a method of controlling an initial release of a beneficial agent from an osmotic delivery device includes the steps of providing a delivery device capsule which encloses a first chamber containing the beneficial agent, a valve member, and a second chamber containing an osmotic agent. The first chamber has a beneficial agent delivery orifice communicating with the external environment. The valve member is initially in a closed position and blocks beneficial agent from passing through the beneficial agent delivery orifice. Upon implantation, the osmotic agent imbibes surrounding fluid to form an osmotic solute which expands and exerts a pressure on the first chamber. The osmotic imbibition of surrounding fluid builds pressure within the osmotic engine until sufficient force is exerted to move the first chamber and valve member, the valve member moving from the closed position to an open position. With the valve in the opened position, beneficial agent contained in the first chamber can pass through the beneficial agent delivery orifice to the external environment.
The present invention provides the advantage of a more controllable beneficial agent delivery rate by preventing expulsion of beneficial agent from the drug reservoir by using a valve to occlude the drug delivery orifice. The valve does not allow beneficial agent to pass through the delivery orifice until sufficient hydraulic pressure exists to displace the beneficial agent from the drug reservoir. Moreover, the present invention retains the valve within delivery device, whereby removing the implant from the patient after delivering the medication allows retrieval of both the valve and the implant device.